Engine emission control systems may utilize various exhaust sensors. One example sensor may be a particulate matter sensor, which indicates particulate matter mass and/or concentration in the exhaust gas. In one example, the particulate matter sensor may operate by accumulating particulate matter over time and providing an indication of the degree of accumulation as a measure of exhaust particulate matter levels. The particulate matter sensor may be located upstream and/car downstream of a diesel particulate filter, and may be used to sense particulate matter loading on the particulate filter and diagnose operation of the particulate filter.
An example PM sensor is shown by Zhang et. al. in US 2015/0355067 A1. Therein, the PM sensor includes a cylindrical protection tube having perforations, and a sensor element is positioned inside the tube facing towards the perforations. The PM sensor is fixed to an exhaust passage downstream of a particulate filter in such a way that the perforations are located on a downstream surface of the protection tube, facing towards a tail end of the exhaust passage. In such a configuration, exhaust gas flowing through the exhaust passage may experience pressure variations along the exterior of the protection tube. For example, a higher static pressure may be created at the downstream surface of the protection tube than along the sides of the protection tube. Because of the higher static pressure at the downstream surface relative to the side surfaces, exhaust gas may be drawn in towards the downstream surface of the PM sensor. In particular, the exhaust gases may be drawn towards the perforations on the downstream surface of the protection tube, and the exhaust may enter the PM sensor via the perforations in a direction opposite to the direction of exhaust flow inside the exhaust passage.
The inventors herein have recognized potential issues with such systems. As an example, the above-mentioned configuration works under the assumption that the exhaust flow direction inside the exhaust passage is constant. Since the sensor is fixed to the exhaust passage, only when the perforations of the protection tube are positioned along a surface that is diametrically opposite to the surface facing the onslaught of exhaust flow will the perforations be coincident with higher static pressure side of the tube. However, if the direction of flow of exhaust inside the exhaust passage changes (e.g., due to changes in engine speed, load, cylinder deactivation, exhaust valve timing, and the like), the perforations may no longer be located on the side with the higher static pressure. In some configurations, the exhaust passage may comprise multiple passages or pathways, some of which are diverging and others that are converging. Together, these pathways may direct exhaust gas through various components of the engine system. If the PM sensor is positioned at an intersection of two orthogonal passages, for example, the direction of exhaust flow may change by 90° when exhaust flow is changed from one passage to the other. In such cases, the perforations on the tube may be in the higher static pressure side when exhaust flows though one of the passages but not when exhaust flows through the second orthogonal passage, for example. If the exhaust flow direction changes by 90° when exhaust flows through the second passage, the perforations on the tube may no longer be located on the higher static pressure side relative to the second passage. Instead, the perforations may now be located on a lower pressure side. Thus, exhaust gas may be pushed away from the perforations thereby reducing the flow of exhaust into the sensor. As a result, the sensitivity of the sensor may be reduced. With reduced sensitivity, the soot sensor may not be able to determine the leakage of the particulate filter in a reliable way. Thus, errors in the sensor may lead to a false indication of DPF degradation and unwarranted replacement of functioning filters.
In one example, the issues described above may be partially addressed by a method comprising, while exhaust is flowing through an exhaust passage, generating an output from a rotatable particulate matter (PM) sensor assembly coupled to the exhaust passage, rotation of the rotatable PM sensor assembly changing as exhaust flow conditions change. In this way, by rotating the PM sensor assembly based on the exhaust flow conditions, an entrance to the PM sensor may be automatically adjusted to be on a side with higher static pressure, thereby increasing the amount of exhaust flow into the PM sensor assembly.
As one example, an exhaust PM sensor assembly may be positioned downstream of an exhaust particulate filter in an exhaust passage. The PM sensor assembly may include a cylindrical housing rotatably mounted to the exhaust passage via a bearing and a sensor element may be positioned within the housing. The housing may additionally include an opening formed only one side, and as such, the opening may be positioned between a pair of perforated flow plates attached to the housing on either side of the opening. The arrangement of the bearing may provide for a free rotation of the PM sensor assembly around a central axis on the housing with reduced friction between the housing and a top surface of the exhaust passage. For example, when a direction of exhaust flow inside the exhaust passage changes by a threshold amount, the PM sensor assembly may rotate inside the exhaust passage in such a way that the opening of the assembly is positioned on a downstream side where the static pressure is higher. In this way, an increased amount of particulates in the exhaust may be directed into the opening towards the sensor element. As such, the rotation of the PM sensor assembly may be one of a passive rotation or an active rotation. During passive rotation, the flow plates attached to the assembly may sense the direction of exhaust flow inside the exhaust passage, and accordingly rotate the assembly via the bearing, for example. During active rotation, the PM sensor assembly may be rotated via a motor coupled to the assembly. Herein, the output of the motor may be adjusted based on the sensed exhaust flow conditions.
The technical effect of rotating the PM sensor assembly inside the exhaust passage based on sensed exhaust flow conditions is that the opening on the housing is automatically moved to a downstream side where the static pressure is higher. Thus, exhaust flowing through the exhaust passage will be diverted around the assembly, and forced to enter the assembly through the opening between the perforated flow plates. In this way, the amount of exhaust entering the assembly may be increased. Exhaust entering though the opening is then directed towards the sensor element that is placed facing towards the opening. Particulates in the exhaust are accumulated across the sensor element. Thus, the amount of exhaust gas and thereby the amount of particulates being deposited on the sensor element may become independent of the incoming exhaust flow direction, thereby measuring PM exiting the particulate filter more accurately and reliably. Further, larger particulates and/or water droplets may be trapped by the flow plates. Therefore, the sensor element may be protected from impingement of water droplets and larger particulates. Overall, these characteristics of the sensor may cause an output of the sensor to be more accurate, thereby increasing the accuracy of estimating particulate loading on a particulate filter.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.